1. Field of the Invention
The present invention relates to ink-jet recording pigment inks (hereinafter simply referred to as “pigment inks”), ink cartridges, and recording units that are suited for ink-jet recording and provide excellent startup properties, anti-sticking properties, and recording head durability after long storage or contact with ink flow passageways. The present invention further relates to ink-jet recording methods, ink-jet recording apparatuses, and methods for stabilizing ink ejection using the above components.
2. Description of the Related Art
Ink-jet recording is a recording method in which ink droplets are ejected from a nozzle by providing energy to the ink to apply the ink to a recording medium such as paper. In recent years, the size of ink droplets ejected from a single nozzle has been reduced to support very high-quality ink-jet recording images at the level of silver-halide photographs. Some currently available ink-jet printers can eject ink droplets with volumes of about 10 pl (picoliters) or less. In addition, support of higher drive frequencies is urgently needed with the increasing demand for higher recording speeds.
In addition to higher definition, ink-jet recording images are required to provide higher fastness (for example, lightfastness). In response to the requirement, pigments are replacing dyes as coloring materials. Japanese Patent Laid-Open No. 56-147871, for example, proposes a recording liquid (ink) at least containing an aqueous medium, a pigment, a polymer dispersing agent, and a nonionic surfactant.
In addition, U.S. Pat. Nos. 5,221,334 and 5,085,698 propose the use of a block copolymer having hydrophilic and hydrophobic segments as a dispersing agent for dispersing a pigment. Currently, however, the study for supporting the creation of images with higher definition at higher speed has not been sufficiently conducted on inks containing a pigment in comparison with dye inks.
Under such circumstances, the present inventors have studied the feasibility of creating ink-jet recording images with higher definition at higher speed using an ink prepared by dispersing a pigment in an aqueous medium with a dispersing agent. In the series of studies, the present inventors have found that not only a pigment and dispersing agent but also a surfactant often plays an important role in terms of the properties of pigment inks, including ejection stability, storage stability, and, particularly, durability for use in thermal heads. Among various surfactants, nonionic and anionic surfactants are preferred and widely used for ink-jet inks.
A pigment ink containing a nonionic surfactant is less susceptible to pH and other ionic materials. This type of surfactant, however, tends to increase the viscosity of the ink. Such a viscous ink causes ejection defects due to partial evaporation of the ink components at the end of a nozzle, thus exhibiting poor startup properties (the ejection stability of an ink from a nozzle suspended temporarily from ejecting the ink). On the other hand, adding the same amount of anionic surfactant to a pigment ink causes no increase in the viscosity of the ink, so that the ink exhibits good startup properties. This type of surfactant, however, causes the ink to be susceptible to pH and other ionic materials. As a result, the ink poses problems such as poor storage stability and clogging due to the reaction with ionic materials from ink flow passageways.
Problems with ink-jet recording pigment inks are summarized below.
1. Startup Properties
For on-demand ink-jet printers, a certain period of suspension from droplet ejection causes relative increases in the concentration of a pigment in the ink remaining at the end of a nozzle with the evaporation of water in the ink to increase the viscosity of the ink at the end of the nozzle. Such a viscous ink cannot be normally ejected, thus exhibiting poor startup properties. In this case, for example, dot placement becomes inaccurate, and the diameter of dots is decreased at the beginning of ejection. Such phenomena are prominent at low humidity, which promotes the evaporation of the ink from the end of the nozzle, and at low temperature, which increases the viscosity of the overall ink. This problem is serious particularly for pigment inks (hereinafter simply referred to as “inks”) because these inks contain a dispersing agent, which is a component that is not contained in dye inks.
To address this problem, for example, the ink in the nozzle is replaced by ejecting the ink outside the printing area if the time during which a recording head faces a recording surface is less than or equal to five seconds in succession. Alternatively, in general, printing is frequently stopped to insert recovery operations in which the ink is sucked and pressurized. Large-format printers, for example, require startup properties better than conventional printers because each scan is long. This problem is more serious for printers having a fixed line head because frequent recovery operations cannot be performed in view of structure.
2. Anti-sticking Properties (Anti-clogging Properties for Nozzles)
Another problem caused by the evaporation of water in the ink at the end of a nozzle is clogging due to the sticking of a coloring material. The clogging occurs when, for example, a printer is left unused for a certain period, a printing head integrated with an ink tank is left detached from a printer, or an ink tank detachable from a printing head is left detached from a printer. This phenomenon is markedly severe and has been difficult to deal with about pigment as opposed to dye, since dye, a colorant itself works as a molecule, is soluble to water or a solvent, on the contrary pigment which associates up to 100 nm in particle size is essentially insoluble to water or a solvent. The evaporation of water in the ink causes relative increases in the concentration of a pigment in the ink to develop a network of the pigment, thus deteriorating anti-clogging properties. Even if no water evaporates, an ink having insufficient dispersibility causes aggregation of the pigment. Such an ink often exhibits poor anti-clogging properties immediately after preparation.
3. Storage Stability
Inks are also required to keep stable ejection with no changes in physical properties, such as aggregation or increased viscosity, and with no effects of changes in pH and a material eluted from ink flow passageways after the inks are left at low or high temperature. The storage stability also greatly affects the anti-clogging properties. Especially, as pigment is uniformly dispersed in ink medium by a dispersing agent, the detachment between pigment and dispersing agent happens or the dispersion of the pigment breaks down and the physical property of the ink rapidly changes when the ink is stored. Inks for use in printers that eject ink droplets with volumes of less than or equal to 10 pl are required to achieve a further improvement in the storage stability because a slight effect of, for example, aggregation or increased viscosity on the individual properties is magnified for such printers.